Similar to LD, I have only cast the lighter weights. They are a nice medium-fast action but would probably not be suited to casting tight loops and a ton of line. If you're looking for something to roll cast 40' I bet it would be great.
I own 3 Batson's and have fished with more than a dozen of these rods...Bro-in-law has built all of them. Agree with the RX6 in a 3-4wt. being a decent stick, but I wouldn't go with less than a RX7 for what you want and the 7+ is well worth the extra money. Your already saving by building it yourself, I would buy the best blank I could afford. I know they have gone way up in price in the last 5 years, but the + series is a much better blank in all weights, IMHO. The RX 8+ 5 wt. gets the most use, lakes, rivers or the salt.